Effects of atmospheric ageing under different temperatures on surface properties of sludge-derived biochar and metal/metalloid stabilization
Graphical abstract
Introduction
Sewage sludge is a byproduct that can be recycled from municipal wastewater treatment process. To improve the water quality in China, the government has established more than 2996 sewage treatment plants since 2012 with a total capacity of 133 million m3 d−1, which produced approximately 30 million tons of sewage sludge each year (moisture content of 80% after dewatering) (Asian Development Bank, 2012). Sludge utilization and disposal has posed a great environmental burden for the authorities and environmental engineers. Pyrolysis, a thermochemical decomposition at elevated temperatures in the absence of oxygen, is a potentially promising method for sludge management, compared to landfilling, incineration (complete combustion of organic substances), and direct agricultural utilization or composting, which often cause secondary pollution due to heavy metals (Waqas et al., 2015). The pyrolysis process can reduce the volume of the bio-solids, eliminate pathogens, improve stability of heavy metals, and transform organic matter into bio-fuel and bio-oil (Waqas et al., 2015, Novak et al., 2016). The resulting biochar byproduct, named as sludge-derived biochar (SDBC), is rich in elemental carbon and nutrients for improving soil fertilization, surface properties, and microbial community abundance (Yao et al., 2010, Ahmad et al., 2017, Igalavithana et al., 2017). The SDBC containing mineral oxides and coexisting Al(III) proved to have good adsorption and stabilization for cationic metal and anionic metalloid species in contaminated soils (such as Pb2+, AsO2−, and CrO42−) with a large amount of exchangeable cations and surface adsorption sites (Fang et al., 2016, Yang et al., 2016, Zhang et al., 2017). As a soil fixation agent, SDBC has aroused increasing attention especially for metal/metalloid-contaminated agricultural and mining soils (Song et al., 2015, Waqas et al., 2015, Zhao et al., 2016).
The surface interactions between SDBC and metals/metalloids are complex and several sorption mechanisms have been proposed for divalent metals (such as Pb, Cu, Cd, Zn, and Ni), including electrostatic attraction with negatively charged surfaces, inner-sphere complex with its surface groups that have a high affinity through covalent bonding, and precipitation or co-precipitation as mineral phases (Lu et al., 2012, Beiyuan et al., 2016, Rajapaksha et al., 2016). The redox reaction between the SDBC and metalloids such as chromate and arsenate are also reported in soils (Zhang et al., 2013b, Zhang et al., 2015, Beiyuan et al., 2017). Although biochars such as SDBC have proven effectiveness for immobilizing metals/metalloids in soils, there are uncertainties regarding the long-term effectiveness as well as change in surface characteristics and sorption behavior of SDBC upon atmospheric exposure at varying temperature.
Wiedner et al. (2013) suggested that the predominance of aromatic carbon with limited amount of negatively charged functional groups on newly produced biochar make it relatively inert. However, previous studies showed that physicochemical characteristics of biochar often change when exposed to the environment. For example, biochar ageing over time was found to be more acidic with a greater cation exchangeable capacity (CEC), more carboxylic surface functional groups (Yao et al., 2010), and higher oxygen content (Ahmad et al., 2012), but a lower zero point of charge, pH, and carbon content (Chen and Chen, 2009, Cao et al., 2009). Ghaffar et al. (2015) revealed that peanut-shell biochar aged in acidic environment showed a decrease in surface area but an increase in the O/C ratio and phthalates sorption capacity. Enriched content of oxygen in the aged biochar was found to significantly increase its ability to stabilize heavy metals (Uchimiya et al., 2011, Qian and Chen, 2013, Qian and Chen, 2014), because of the increase in oxygen-containing functional groups. Therefore, exposure of biochar to atmospheric air during storage and application may introduce polar functional groups on the biochar surfaces, which, in turn, affect the bioavailability and mobility of metals/metalloids.
Thus far, most ageing experiments were performed by mixing biochar with soil over time, while incubation experiment of biochar by itself was rare. Hence, this study exposed the SDBC to the air at 4, 22, and 45 °C for 120 d to investigate the effects of the ageing process on the surface properties of SDBC and its stabilization performance for metal (Pb(II)) and metalloids (As(III) and Cr(VI)) with the aid of microscopic and spectroscopic analysis.
Section snippets
SDBC production and exposure to air under different temperatures
The SDBC used in this study was prepared through pyrolyzing municipal sewage sludge, which was collected from Lijiao Sewage Treatment Plant in Guangzhou (23°20′N, 113°30′E) and dried for 12 h in an oven at 80 °C. The sludge pyrolysis was conducted in a LTKC-6-12 pipe oven (Blue Sky, Hangzhou, China) at 400 °C for 2 h, which are the optimal conditions based on our previous studies (Zhang et al., 2013b, Zhang et al., 2017). The elemental compositions of the sludge and the produced SDBC,
Change of surface properties of SDBCs during atmospheric ageing
As illustrated in Fig. 1, the surface acidity of SDBC increased with the ageing time, and SDBC exposed at 22 °C often accommodated the most acid groups on the surface. Yet, the surface basic groups of SDBCs decreased with the aging time especially at 45 °C. As shown in Table S1, the fresh and 30 d-aged SDBC samples were found to have higher surface basicity than surface acidity, while the 120 d-aged SDBC samples under all studied temperatures showed the opposite. Leon et al. (1992) attributed
Conclusions
To understand the effects of ageing on the sludge-derived biochar itself during storage and soil application, we characterized the 30- to 120-d aged SDBC and evaluated their sorption capacity for Pb(II), Cr(V), and As(III). The aged SDBC displayed an increase in surface acidity and cation exchange capacity, which were due to the enriched surface density of carboxyl groups. Yet, there was a decrease in the alkalinity and the amount of Fe(II) species due to oxidation of the SDBC surface during
Acknowledgement
The authors wish to thank the National Natural Science Foundation of China (Project no. 41272383), Shenzhen Municipal Science and Technology Plan Project (Contract no. JCYJ20160519095007940), and Hong Kong Research Grants Council (PolyU 15222115) for the financial support of this study.
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